JP2015137474A - work vehicle - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a work vehicle, in which a significant pump loss reduction effect can be obtained when steering operation is performed.SOLUTION: A work vehicle comprises open center type first control valves 10a and 10b for controlling a flow rate of hydraulic fluid fed from a variable capacity hydraulic pump 4 to hydraulic actuators 11a and 11b for driving a work device 50, a second control valve 8 for controlling a flow rate of hydraulic fluid fed from the hydraulic pump to a steering cylinder 9 for steering wheels 61, a priority valve 6 for prioritizing feeding of hydraulic fluid to the second control valve, a throttle 12 provided downstream of a center bypass line of the first control valve, control levers 14a and 14b for changing a spool position of the first control valve, and a flow controller 5 for controlling a discharge flow rate of the hydraulic pump within a range that is smaller than that under control by the control levers and allows the steering cylinder to be driven when the control levers are not operated.

Description

  The present invention relates to a work vehicle including a hydraulic circuit capable of preferentially supplying hydraulic oil discharged from a hydraulic pump to a steering actuator.

  As a technique related to a work vehicle such as a wheel loader, there is a hydraulic drive device disclosed in Japanese Patent No. 4446822.

  The hydraulic drive device includes a variable displacement hydraulic pump and a steering control for controlling a flow rate of pressure oil (hydraulic oil) supplied from the hydraulic pump to a hydraulic actuator (steering actuator: for example, a steering cylinder) related to steering. A valve (control valve), an open center type (center bypass type) work device control valve (control valve) for controlling the flow rate of pressure oil supplied from the hydraulic pump to a hydraulic cylinder related to the work device, A priority valve for preferentially diverting the hydraulic oil from the hydraulic pump to the steering control valve during steering operation, and a throttle provided downstream of the center bypass line of the work device control valve, As the pressure generated by the throttle increases, the hydraulic pump discharges. To the pressurized oil flow rate (pump discharge flow rate) has a configuration for controlling to decrease.

  As a result, the hydraulic drive device according to the above-mentioned document can reduce the loss (pump loss) caused by the drive of the hydraulic pump while preferentially securing the pressure oil flow rate required for driving the hydraulic actuator related to steering over the other hydraulic actuators. Mitigating.

Japanese Patent No. 4446822

  In the hydraulic drive device according to the above document, the throttle pressure increases with an increase in the flow rate of pressure oil passing through the throttle (throttle passage flow rate), and the relationship between the throttle pressure and the pump discharge flow rate (throttle characteristics) And pump control characteristics) are characteristics in which the pump discharge flow rate decreases as the throttle pressure increases.

  The difference between the pump discharge flow rate and the throttle passage flow rate indicates the flow rate of pressure oil used for driving a hydraulic actuator such as a working device including a steering cylinder. In the hydraulic drive device according to the above literature, when both the steering lever and the operation lever of the working device are not operated, there is no flow rate used to drive the hydraulic actuator (that is, no hydraulic oil is supplied to each hydraulic actuator) The difference is zero between the pump discharge flow rate and the throttle passage flow rate, that is, the pump discharge flow rate and the throttle passage flow rate are balanced. At this time, the throttle passage flow rate and the throttle pressure are maximum values, and the pump discharge flow rate is set to the minimum value. When the steering operation is performed from this state, the pressure oil required for the steering actuator is preferentially guided by the priority valve, so that the flow rate through the throttle and the pressure in the throttle decrease from the maximum values, and the pump discharge flow rate increases from the minimum value. Is done. The flow rate through the throttle does not contribute to the drive of the hydraulic actuator and is a surplus flow rate, which contributes to pump loss. However, in the hydraulic drive device according to the above document, the flow rate through the throttle decreases when the flow rate to the steering actuator increases due to the steering operation. Therefore, it can be said that there is a pump loss reduction effect from the viewpoint of the throttle passage flow rate.

  However, the characteristics of “pump flow rate change” with respect to “throttle pressure change” in ordinary work vehicles including the wheel loader described in the above document are set to be compatible with work devices that require a large flow rate of pressure oil. Specifically, the setting is such that the pump discharge flow rate increases remarkably with respect to a slight decrease in the throttle pressure. Therefore, when the pressure oil is supplied to the steering actuator that requires a relatively small flow rate compared with the hydraulic actuator of the working device, the reduction amount of the flow rate through the throttle is small, and the pump discharge flow rate is determined according to the throttle pressure. The effect of reducing pump loss is limited only by controlling. Therefore, improvement of pump loss during steering operation is desired.

  The present invention has been made in view of the above points, and an object thereof is to provide a work vehicle capable of obtaining a large pump loss reduction effect when a steering operation is performed.

  The present invention includes a plurality of means for achieving the above object. For example, a variable displacement hydraulic pump and a hydraulic actuator for driving a working device are supplied from the hydraulic pump. An open center type first control valve for controlling the flow rate of the pressure oil and a second control for controlling the flow rate of the pressure oil supplied from the hydraulic pump to the steering actuator for steering the wheel. A valve, a priority valve for preferentially supplying pressure oil from the hydraulic pump to the second control valve, a throttle provided downstream of a center bypass line in the first control valve, and the first control In a work vehicle comprising an operating device for changing a spool position of a valve, the steering device is smaller when the operating device is not operated than when the operating device is operated, and the steering The drive is range of actuators, it shall comprise a flow control device for controlling the discharge flow rate of the hydraulic pump.

  ADVANTAGE OF THE INVENTION According to this invention, the pump loss while the operating device for working devices is non-operating can be reduced.

The side view of the wheel loader concerning an embodiment of the invention. FIG. 2 is a schematic diagram of a hydraulic circuit of a hydraulic drive device mounted on the wheel loader shown in FIG. 1. The figure which showed the characteristic which shows the relationship between the pressure which the shuttle valve 17 outputs, and the discharge flow volume of the hydraulic pump 4, and the characteristic which shows the relationship between the pressure which generate | occur | produces in the throttle 12, and a throttle passage flow rate on one graph. The figure which showed three balance points U, V, and W when the operation lever 14 is operated on the graph of FIG.

  First, before describing the embodiment of the present invention, main features included in the work vehicle according to the embodiment of the present invention will be described.

  (1) A work vehicle (for example, a wheel loader) according to an embodiment of the present invention described later includes a variable displacement hydraulic pump and a hydraulic actuator (for example, a front work device) for driving a work device (for example, a front work device). An open center type first control valve for controlling the flow rate of pressure oil supplied from the hydraulic pump to a lift cylinder and a bucket cylinder, and an operating device for changing the spool position of the first control valve (For example, an operation lever), a second control valve for controlling the flow rate of pressure oil supplied from the hydraulic pump to a steering actuator (for example, a steering cylinder) for steering a wheel, and the second A steering device (for example, a steering wheel) for changing the spool position of the control valve, and the hydraulic position when the steering device is operated. A work vehicle comprising: a priority valve (for example, a priority valve) for preferentially supplying pressure oil from the pump to the second control valve; and a throttle provided downstream of a center bypass line in the first control valve. The control device includes a flow rate control device that controls the discharge flow rate of the hydraulic pump in a range that is smaller than when the operation device is operated and in which the steering actuator can be driven when the operation device is not operated.

  In a work vehicle including a flow rate control device that controls the discharge flow rate of a hydraulic pump according to the pressure of the throttle and the priority valve, when the steering device is operated, the priority valve preferentially pressurizes the second control valve. Since oil is supplied, even if the operating device is in a non-operating state, the flow rate of the pressure oil flowing through the center bypass line of the first control valve is reduced, and the pressure generated in the throttle is reduced. In this regard, in a conventional work vehicle in which control (so-called negative control control (negative control)) is performed to increase the discharge flow rate of the hydraulic pump as the throttle pressure decreases, the "change in throttle pressure" The characteristic of `` change in pump discharge flow rate '' is set to be compatible with hydraulic actuators of work equipment that require large flow rate of pressure oil. The setting is such that the pump discharge flow rate increases significantly. Therefore, the amount of reduction in the flow rate through the throttle when supplying pressure oil to a steering actuator that requires a relatively small flow rate compared with the hydraulic actuator of the working device is small (for example, in the example of FIG. 3 described later). Even if the pump discharge flow rate increases from the state of point A by the flow rate (qs) necessary for driving the steering actuator and moves to point B ′ (balance point in the prior art), the amount of decrease in the flow rate through the throttle Is only Δq.), The pump loss reduction effect was limited only by controlling the pump discharge flow rate according to the pressure of the throttle.

  On the other hand, according to the work vehicle according to the present embodiment including the flow rate control device, when the operation device is not operated, the operation vehicle is smaller than when the operation device is operated, and the steering actuator (for example, the steering cylinder) ), The discharge flow rate of the hydraulic pump is controlled to the extent that it can be driven, so that the discharge flow rate of the hydraulic pump can be prevented from greatly increasing when the operation device is not operated, and the startability of the steering operation is ensured. However, the excessive flow rate when the operating device is not operated is reduced, and the pump loss can be significantly reduced.

  The hydraulic pump discharge flow rate when the operation device is not operated is smaller than that when the operation device is operated and the steering actuator can be driven. The discharge flow rate of the hydraulic pump may be changed as appropriate according to the pressure value.

  Further, when the operation device is not operated, the discharge flow rate of the hydraulic pump may be held at a constant value that is smaller than that when the operation device is operated and that allows the steering actuator to be driven. Also in this case, since the discharge flow rate of the hydraulic pump can be maintained at a smaller value than when the operating device is operated when the operating device is not operated, the pump loss can be reduced. Further, specific examples of values that can be used as “the constant value” include a flow rate (qs) required for driving the steering actuator, and maintaining the operation of the work vehicle when the operation device and the steering device are not operated. There is a total value with the minimum required flow rate (required minimum flow rate) qm. With this setting, the pump loss can be minimized while maintaining the operation of the work vehicle.

  (2) In the above (1), it is preferable that the flow rate control device controls a value of a discharge flow rate of the hydraulic pump so as to increase with an increase in an operation amount of the operation device when the operation device is operated. . Thus, when the value of the discharge flow rate of the hydraulic pump is increased with the increase of the operation amount of the operation device, the discharge flow rate of the hydraulic pump is increased according to the increase of the operation amount of the operation device. It is possible to ensure good responsiveness of the hydraulic actuator when the working device is operated via the actuator.

  Hereinafter, embodiments of the present invention will be specifically described with reference to the drawings. FIG. 1 is a side view of a wheel loader according to an embodiment of the present invention. In addition, the same code | symbol is attached | subjected to the same part in each figure, The description of the said same part may be abbreviate | omitted.

  The wheel loader 100 of FIG. 1 includes a vehicle body 110 and a hydraulically-driven work device (front work device) 50 attached in front of the vehicle body 110. The vehicle body 110 employs an articulated steering type (vehicle body refraction type), and a front vehicle body (front frame) 111 having wheels 61 (one set of front wheels 61a and one set of rear wheels 61b) mounted on the left and right, respectively. And a rear vehicle body (rear frame) 112 are connected by a pin 115 having a substantially vertical axis. Although not shown in FIG. 1, steering cylinders 9 (see FIG. 2) are respectively arranged on the left and right sides of the pin 115 so as to connect the front vehicle body 111 and the rear vehicle body 112. The steering cylinder 9 is a pair of hydraulic cylinders (hydraulic actuators) that are driven to extend and contract to steer the wheels 61. When the steering wheel 20 (see FIG. 2), which is a steering device installed in the cab 116, is operated from the neutral position, one of the two steering cylinders 9 extends and the other contracts, so that the rear vehicle body 112 and the front The partial vehicle body 111 refracts (turns) around the pin 115. In FIG. 2, only one steering cylinder 9 is shown in a simplified manner.

  Here, an articulated steering type wheel loader will be described as an example, but a hydraulic actuator is used to steer the wheel 61 (for example, an all hydraulic front wheel steering type or a rear wheel steering type). If so, the present invention can be applied to a steering system other than the articulate steering system.

  On the rear vehicle body 112, a driver's cab 116 is mounted in the front and an engine chamber 117 is mounted in the rear. The engine chamber 117 houses a variable displacement hydraulic pump 4 (see FIG. 2) driven by a diesel engine (not shown) as a prime mover.

  The work device 50 is lifted and retracted to drive the lift arm 121, the bucket 122, the lift cylinder 11 a that is a hydraulic cylinder (hydraulic actuator) that is driven to extend and contract to drive the lift arm 121, and the bucket 122. And a bucket cylinder 11b which is a hydraulic cylinder (hydraulic actuator). One lift arm 121 and one lift cylinder 11a are provided on the left and right of the front vehicle body 111, respectively, but the right lift arm 121 and the lift cylinder 11a hidden in FIG. 1 are omitted.

  The lift arm 121 rotates in the vertical direction (up and down movement) as the lift cylinder 11a extends and contracts. The bucket 122 rotates in the vertical direction (dump operation or cloud operation) with the expansion and contraction drive of the bucket cylinder 11b. The illustrated wheel loader 100 includes a Z link type (bell crank type) link mechanism for operating the bucket 122.

  FIG. 2 is a schematic diagram of a hydraulic circuit of a hydraulic drive device mounted on the wheel loader shown in FIG. The hydraulic drive device shown in this figure is a variable displacement hydraulic pump 4 and a flow control device (pump flow control) that controls the discharge flow rate of the hydraulic pump 4 by changing the displacement (tilt angle) of the hydraulic pump 4. Piston) 5, a priority valve (steering priority valve) 6 connected to the discharge side of the hydraulic pump 4 and having a priority output port and a non-priority output port, and control so that the discharge pressure of the hydraulic pump 4 does not exceed a specified pressure A control valve for steering (controlling the flow rate and direction of the pilot switching system) that is connected to the relief valve 7 and the priority output port of the priority valve 6 and controls the flow rate of the pressure oil supplied from the hydraulic pump 4 to the steering cylinder 9 A directional control valve) 8 and a steering cylinder (steering actuator) 9 driven and controlled by the steering control valve 8. A steering wheel (steering device) 30 for changing the spool position of the steering control valve 8 and a non-priority output port of the priority valve 6 are connected to the lift cylinder 11a and the bucket cylinder 11b from the hydraulic pump 4. Center bypass type (open center type) two work device control valves (directional control valves for controlling the flow rate and direction of the pilot switching method) 10a, 10b for controlling the flow rate of the pressurized oil, and for each work device A lift cylinder 11a and a bucket cylinder 11b (plural working device actuators) driven and controlled by the control valves 10a and 10b, respectively, generate pressure (pilot pressure) corresponding to the operation amount, and the working device control valve 10a is generated by the pressure. An operating lever (operating device) 14a for changing the spool position of the An operation lever (operation device) 14b that generates a pressure corresponding to the amount and changes the spool position of the work device control valve 10b by the pressure, and downstream of the center bypass line of the two work device control valves 10a and 10b. A throttle 12 that is arranged and generates a negative control pressure (negative control pressure) that is a control pressure for inverse proportional control with respect to the pump flow control device 5, and is connected in parallel with the throttle 12, and a negative control pressure generated at the throttle 12 is defined. And a relief valve 13 for regulating the pressure so as not to exceed the pressure.

  The shuttle valve 15 selects the maximum pilot pressure output from the two operation levers 14 a and 14 b and outputs the maximum pressure to the pressure reducing valve 16. The pressure reducing valve 16 reduces the pilot pressure from a hydraulic source (for example, a pilot pump) according to the magnitude of the pressure output from the shuttle valve 15 and outputs the pressure after the pressure reduction to the shuttle valve 17. The pressure output from the pressure reducing valve 16 to the shuttle valve 17 is set so as to decrease as the pressure output from the shuttle valve 15 increases. The shuttle valve 17 selects a higher pressure from the pressures output from the pressure reducing valve 16 and the throttle 12, and outputs the pressure on the high pressure side to the flow control device 5 as a control pressure. The flow rate control device 5 controls the tilt angle of the hydraulic pump 4 so that the discharge flow rate of the hydraulic pump 4 is controlled to a value corresponding to the magnitude of the pressure output from the shuttle valve 17. Next, the relationship between the pressure value output from the shuttle valve 17 and the discharge flow rate of the hydraulic pump 4 in the present embodiment will be described with reference to FIG.

  FIG. 3 is a graph showing the characteristics indicating the relationship between the pressure output from the shuttle valve 17 and the discharge flow rate of the hydraulic pump 4, and the characteristics indicating the relationship between the pressure generated in the throttle 12 and the flow rate through the throttle. is there. As shown in this figure, the pressure of the throttle 12 has a characteristic that increases as the flow rate through the throttle increases, and the pump discharge flow rate has a characteristic that decreases as the output pressure of the shuttle valve 17 increases. .

  The difference between the pump discharge flow rate and the throttle passage flow rate indicates the flow rate used to drive the hydraulic actuators 11a, 11b, 9 related to the work device 50 and the like, and the operation levers 14a, 14b (work device control valves 10a, 10b) and the steering wheel. When the wheel 20 (steering control valve 8) is in a neutral state, there is no flow rate used to drive the hydraulic actuators 11a, 11b, and 9, so that the pump discharge flow rate is balanced at a point A that intersects the throttle passage flow rate curve.

  Thus, when the work device control valves 10a, 10b and the steering control valve 8 are in the neutral position, the flow rate (throttle passage flow rate) passing through the center bypass line related to the work device control valves 10a, 10b is the maximum, At the same time, the pressure P12 generated in the throttle 12 (hereinafter sometimes referred to as negative control pressure) is also maximum. When the operation levers 14a and 14b are in the neutral position, the pressure output from the shuttle valve 15 is the minimum, and as a result, the output pressure P16 from the pressure reducing valve 16 is the maximum. That is, “Ps = P12 = P16” in FIG.

  The pump discharge flow rate Qs at the point A is a standby flow rate, and in this embodiment, the flow rate qs necessary for driving the steering actuator, and maintaining the operation of the work vehicle when the operation levers 14a and 14b and the steering wheel 20 are not operated. A predetermined value larger than the total value of the minimum required flow rate (required minimum flow rate) qm is set. The standby flow rate Qs may be equal to or greater than the total value of qs and qm, but a value close to the total value is preferable from the viewpoint of reducing pump loss.

  As shown in the graph of FIG. 3, the throttle passage flow rate gradually increases as the pressure of the throttle 12 increases, passes through the standby flow rate Qs at the pressure Ps, and finally approaches a value slightly larger than the standby flow rate Qs. And is set to converge.

  On the other hand, the pump discharge flow rate takes a maximum value when the output pressure of the shuttle valve 17 is equal to or lower than the pressure P1 lower than Ps, and decreases monotonically between P1 and P2 (P2> Ps). It is set to hold the minimum value. Ps is a value smaller than P2, and as described above, the output pressure of the shuttle valve 17 when the operation levers 14a and 14b and the steering wheel 20 are not operated is Ps, so the pressure does not exceed Ps. The minimum value of the substantial pump discharge flow rate in the form is Qs.

  As described above, the pump discharge flow rate is determined based on the output pressure of the shuttle valve 17. That is, the pump discharge flow rate is determined by the larger one of the two pressures of the pressure P12 of the throttle 12 and the output pressure P16 of the pressure reducing valve 16 input to the shuttle valve 17. For example, even if the pressure P12 of the throttle 12 is less than Ps, if the output pressure P16 of the pressure reducing valve 16 is Ps, the pump discharge flow rate is held at Qs.

  Next, the operation of the work vehicle configured as described above will be described. In the hydraulic drive apparatus shown in FIG. 2, the pressure oil discharged from the hydraulic pump 4 flows to the priority valve 6. The priority valve 6 includes a steering hydraulic circuit 70 connected to two output ports (priority output port and non-priority output port), and a steering hydraulic circuit 70 connected to the priority output port among the hydraulic circuit 60 for work implements. This is a known steering-priority diverter valve that diverts pressure oil in preference to the steering oil. When the steering wheel 20 is operated, pressure oil having a flow rate necessary for steering the wheel 61 is preferentially flowed to the steering hydraulic circuit 70. The other pressure oil is caused to flow to the working machine hydraulic circuit 60. In addition, when the steering wheel 20 is not operated (that is, when the steering wheel 20 is in the neutral position), the priority valve 6 allows the entire amount of pressure oil to flow to the working machine hydraulic circuit 60.

  Each work machine control valve (each direction switching valve) 10a, 10b is a pilot switching type valve in which the spool position is changed by the pilot pressure output from the operation levers 14a, 14b, and the operation levers 14a, 14b are neutral. At the position (when the working device 50 is not operated), each working device control valve 10a, 10b is in the neutral position, the flow rate passing through the center bypass line is maximized, and the negative control pressure P12 is also maximized. When the operation levers 14a and 14b are in the neutral position, the output pressure from the shuttle valve 15 is further minimized and the output pressure P16 from the pressure reducing valve 16 is maximized.

  When at least one of the operation levers 14a and 14b is operated from this state, a pressure corresponding to the operation amount is output to the work machine control valve 10, the spool position is switched, and the flow rate passing through the center bypass line (passing through the throttle) The negative control pressure P12 is lower than when the operation levers 14a and 14b are in the neutral position. At the same time, the pressure output from the operation levers 14a and 14b to the shuttle valve 15 increases according to the operation amount, so the output pressure P16 from the pressure reducing valve 16 decreases. When the operation amount becomes maximum (full stroke), the center bypass line is cut off, the negative control pressure P12 becomes zero, and the output pressure P16 from the pressure reducing valve 16 becomes minimum.

  At this time, a high pressure is selected from the negative control pressure P12 and the output pressure P16 by the shuttle valve 17 and is sent to the pump flow control device 5 as a pump control pressure. As described above, the output pressure P16 from the pressure reducing valve 16 is maximum when the operation levers 14a and 14b are in a neutral state, decreases as the operation amount of the operation levers 14a and 14b increases, and becomes minimum when the operation amount is maximum. The transition of the same tendency as the negative control pressure P12 by the diaphragm 12 is shown. Therefore, in the present embodiment, the discharge flow rate of the hydraulic pump 4 is the minimum when the operation levers 14a and 14b are in the neutral state, increases according to the operation amount, and finally becomes predetermined as the operation amount increases. It can be seen that there is a tendency to converge to the value (maximum value). Thus, when the pump discharge flow rate is increased with the increase in the operation amount, it is possible to ensure good responsiveness of the hydraulic actuators 11a and 11b when operating the work device 50 via the operation levers 14a and 14b.

  Next, a change in the pump discharge flow rate when the operation levers 14a and 14b (working device 50) are not operated and only the steering hole 30 is operated will be described.

  When the operation levers 14a and 14b and the steering wheel 20 are not operated (neutral state), all of the hydraulic oil discharged from the hydraulic pump 4 passes through the center bypass line related to the work device control valves 10a and 10b. Is the maximum, and the negative control pressure P12 generated at the throttle 12 is also the maximum value Ps. When the operation levers 14a and 14b are in the neutral position, the pressure output from the shuttle valve 15 is the minimum, and as a result, the output pressure P16 from the pressure reducing valve 16 becomes the maximum value Ps. Thus, the pump discharge flow rate is balanced at the point A shown in FIG.

  When the steering operation is performed from the state of being balanced at point A, the pressure oil from the hydraulic pump 4 is sent to the steering cylinder 9 via the steering control valve 8 by the priority valve 6, and therefore the work device control valve 10. The flow rate passing through the center bypass line decreases, and the negative control pressure P12 generated in the throttle 12 decreases. In this case, in the case of a conventional work vehicle that does not include the shuttle valves 15 and 17 and includes the flow rate control device 5 that controls the pump discharge flow rate only according to the negative control pressure, the flow rate control device 5 causes the hydraulic pump 4 to decrease due to the decrease in the negative control pressure P12. The discharge flow rate should be increased. Specifically, when the steering operation is performed from the state of the point A in FIG. 3, the discharge flow rate of the hydraulic pump 4 is increased by the flow rate qs necessary for the steering, and the point B ′ in FIG. You will be balanced. That is, even if the operation levers 14a and 14b are not operated, the negative control pressure is reduced to a value smaller than Ps by the steering operation. Therefore, the pump discharge flow rate is set to a value larger than the flow rate Qs as in the operation of the operation levers 14a and 14b. It will change.

  In contrast, in the present embodiment, the control pressure output from the shuttle valve 17 to the flow control device 5 is the negative control pressure P12 generated at the throttle 12 and the output pressure P16 from the pressure reducing valve 16 (in this case, the maximum value). Since P16 (the maximum value Ps) on the high pressure side is selected, the control pressure does not become smaller than Ps even if the negative control pressure P12 decreases from Ps, and the discharge flow rate of the hydraulic pump 4 corresponds to the corresponding flow rate Qs. It won't be bigger. That is, regardless of whether or not the steering operation is performed, P16 is always held at the maximum value Ps when the operation levers 14a and 14b are not operated, so that the output pressure from the shuttle valve 17 does not become smaller than Ps, and the pump discharge flow rate. Is held at a value smaller than that when the operating levers 14a and 14b are operated. At this time, the flow rate qs of the discharge flow rate Qs is supplied to the steering actuator 9, the pump discharge flow rate is balanced at the point B in FIG. 3, and the flow rate (throttle) passing through the center bypass line related to the work device control valve 10 (Passing flow rate) can be significantly reduced than before. In the example of FIG. 3, since the throttle passage flow rate can be reduced by ΔQ as compared with the conventional case, the pump loss can be remarkably reduced.

  As described above, in the present embodiment, when the operation levers 14a and 14b are in the non-operation state, even when the steering wheel 20 is operated, the values at the time of operation of the operation levers 14a and 14b (that is, on the graph of FIG. 3). The pressure is less than Ps and the value set as the pump discharge flow rate (that is, a value larger than Qs) is smaller than the value set for steering operation (that is, Qs). While ensuring good startability, the excessive flow rate during steering operation can be reduced more than before, and pump loss can be reduced.

  In the above example, a case has been described in which the pump discharge flow rate is held at a constant value Qs when the operation levers 14a and 14b are not operated. If it is in the range where 9 can be driven, even if the discharge flow rate of the hydraulic pump is appropriately changed within the range, the pump loss reducing effect can be obtained in the same manner as described above. Further, at that time, the pump discharge flow rate may be appropriately changed according to the negative control pressure P12.

  Incidentally, the point of the present invention described above is that the discharge flow rate of the hydraulic pump 4 is not increased more than necessary when the two operation levers 14a and 14b are not operated. As long as the operation state is detected and the control for changing the discharge flow rate of the hydraulic pump 4 can be interrupted while the detection result is non-operation, the same effect can be obtained with other configurations. As a means for detecting the operation state of the operation levers 14a and 14b, for example, a displacement detector for the operation levers 14a and 14b such as a potentiometer, a pressure sensor when the operation levers 14a and 14b output a hydraulic signal, or an operation lever When 14a and 14b output electrical signals, current / voltage sensors can be used. And as a means for interrupting the pump discharge flow rate control, for example, a control or device for cutting off the control signal to the flow rate control device 5 while the operation levers 14a and 14b are not operated, or a state where the operation levers 14a and 14b are not operated. In addition, a device (mechanical stopper) for mechanically fixing the tilt of the hydraulic pump 4 can be used.

  In the above description, the case where the pump loss is reduced by interrupting the increase control of the pump discharge flow rate based on the negative control pressure P12 when the two operation levers 14a and 14b are not operated has been described. Even when the operation levers 14a and 14b are operated, there is a possibility that the pump loss can be reduced by the same action as described above. More specifically, even when the operation amount of the operation levers 14a, 14b is increased, the output pressure P16 of the pressure reducing valve (hereinafter sometimes referred to as positive control pressure (positive control pressure)) rather than the negative control pressure P12. If becomes larger, the positive control based on the positive control pressure P16 is executed preferentially over the negative control based on the negative control pressure P12, and there is a possibility that the pump loss can be reduced by the same operation as described above. Therefore, next, the magnitude relationship between the two pressures P12 and P16 when the operation amount of the operation levers 14a and 14b is increased will be considered. In the following description, when only one of the two operation levers 14a and 14b is shown, the subscripts a and b may be omitted and referred to as “operation lever 14”.

  When the operation amount of the operation lever 14 is increased, the positive control pressure P16 decreases as the operation amount increases, so that the pump discharge flow rate increases. On the other hand, with respect to the negative control pressure P12, the area of the center bypass flow path provided in the control valves 10a and 10b is reduced as the operation amount of the operation lever 14 is increased. The pressure P12 decreases and the pump discharge flow rate increases. When the operation lever 14 is operated, in principle, it is not uniquely determined which of the positive control pressure P16 and the negative control pressure P12 is higher. However, it can be clarified under certain conditions (three conditions described later).

  Here, FIG. 4 will be first described before consideration under the conditions. FIG. 4 shows three balance points U, V, and W on the graph of FIG. 3 in order to explain the relationship between the pressures P12 and P16 when the operation lever 14 is operated in the wheel loader according to the present embodiment. FIG.

  In this figure, the balance point at which the positive control pressure P16 becomes Pu at a certain lever operation amount is U, and the pump discharge flow rate at that time is Q1. At the point U, the pump discharge flow rate is limited to Q1 or less and does not exceed Q1. At this time, the bypass flow rate (throttle passage flow rate in FIG. 4) governing the negative control is Qb, and here, it is assumed that the working device 50 operates at a flow rate of Q1-Qb. Hereinafter, a flow rate obtained by subtracting the bypass flow rate from the pump discharge flow rate (for example, Q1-Qb) may be referred to as an operation flow rate. At this time, first, considering that “the bypass flow rate (throttle passage flow rate) is uniquely determined according to the operation amount of the operation lever 14”, the negative control pressure P12 also becomes Pu, and eventually the pump discharge flow rate is determined according to the positive control pressure P16. Will be controlled.

  However, the flow rate of the pressure oil including the bypass flow rate is not determined only by the throttle opening degree (open area of the center bypass flow path) of the control valves 10a and 10b defined by the operation amount of the operation levers 14a and 14b. It also changes depending on the state of negative pressure.

  For example, even if the operation amount of the operation lever 14 is the same, when the load pressure is high and the pump pressure is high, the bypass flow rate increases and the negative control pressure P12 becomes relatively high. Using the U point as a reference, when the load pressure is high, the negative control pressure P12 rises from the positive control pressure P16 (Pu) and moves from the U point to the V point, reducing the pump discharge flow rate and the operating flow rate. Less. Hereinafter, this example may be referred to as “Exemplary V”.

  On the contrary, even if the operation amount of the operation lever 14 is the same, when the load pressure is low and the pump pressure is low, the bypass flow rate is reduced and the negative control pressure P12 is relatively low. With reference to point U, when the load pressure is low, the negative control pressure P12 decreases from Pu and the bypass flow rate (throttle passage flow rate) decreases, but the pump is driven by the positive control pressure 16 (Pu) higher than the negative control pressure P12. Since the flow rate is limited to Q1, the operation flow rate is relatively increased by the decrease of the bypass flow rate. At this time, the pump discharge flow rate does not increase as much as when only negative control is performed, and depending on the design, there is a possibility that the pump loss can be reduced even during the operation 50 according to the present embodiment. Hereinafter, this example may be referred to as “Exemplary W”.

  Based on this, the magnitude relationship between the positive control pressure P16 and the negative control pressure P12 under the following three conditions 1 to 3 will be considered.

<Condition 1: Lift and bucket combined operation of work equipment>
Since the positive control pressure P16 is determined by the maximum operation amount of the two operation levers 14a and 14b, the combined operation in which the two operation levers 14a and 14b are operated together is not affected by the smaller operation amount. . However, since the operation amount of the operation lever 14 with a small operation amount affects the negative control pressure, the following (1) and (2) can be understood according to the operation of the operation lever 14.

  (1) When the operation amount of the operation lever 14 with a small operation amount is smaller than a state in which there is an operation amount (assumed to be a U point in FIG. 4), the bypass circuit is opened, and the bypass flow rate increases. As a result, the negative control pressure P12 becomes higher than Pu, the negative control is enabled, and the pump flow rate is reduced (that is, the state is the same as in Example V).

  (2) Contrary to the above, when the operation amount of the operation lever 14 having the smaller operation amount becomes large (in a range not larger than the other operation amount), the bypass circuit is throttled, so that the bypass flow rate is reduced. As a result, the negative control pressure P12 becomes lower than Pu. However, since positive control is enabled, the pump flow rate is maintained at Q1, and the operation diversion increases by the amount by which the bypass flow rate is reduced (that is, the state is the same as in Example W). .

<Condition 2: Load pressure fluctuation>
Next, as described above, when the load pressure fluctuates while only one of the two operation levers 14a and 14b is held at a predetermined operation amount, the following (1), (2 )

  (1) If the load pressure increases in this state, the bypass flow rate increases, the negative control becomes effective, and the pump flow rate decreases.

  (2) Contrary to the above, when the load pressure decreases, the bypass flow rate decreases, but the pump flow rate does not increase, and the operating flow rate increases by the amount by which the bypass flow rate decreases, so the state is the same as in Example W.

<Condition 3: Steering operation>
Next, when the operation amount of the steering wheel 20 varies with only one of the two operation levers 14a and 14b being held at a predetermined operation amount, the following ( 1) and 2) are understood.

  (1) If the steering flow rate increases in this state, the bypass flow rate decreases, but the pump flow rate does not increase, and the operation flow rate increases by the amount by which the bypass flow rate decreases, so the state is the same as in Example W.

  (2) Contrary to the above, when the steering flow rate is reduced, the bypass flow rate is increased, and the pump flow rate is reduced by the negative control, so that the state is the same as in Example V.

  As described above, according to the present embodiment, not only when the working device 50 is not operated, but also when the working device 50 is operated, the negative control pressure P12 becomes smaller than the positive control pressure P16 (for example, In the case of the condition 1 (2), the condition 2 (2), the condition 3 (1)), there is a possibility that the pump loss can be reduced more than before.

  In the above description, the wheel loader including a priority valve that supplies pressure oil to the steering cylinder preferentially has been described as an example. However, the hydraulic pressure that can preferentially supply the hydraulic oil discharged from the hydraulic pump to the steering actuator. As long as the circuit is provided, the present invention can be applied to other work vehicles.

  Further, the present invention is not limited to the above-described embodiment, and includes various modifications within the scope not departing from the gist thereof. For example, the present invention is not limited to the one having all the configurations described in the above embodiment, and includes a configuration in which a part of the configuration is deleted.

  4 ... Variable displacement hydraulic pump, 5 ... Pump flow rate control device (flow rate control device), 6 ... Priority valve (priority valve), 8 ... Steering control valve (second control valve), 9 ... Steering cylinder (steering actuator) DESCRIPTION OF SYMBOLS 10 ... Work device control valve (1st control valve), 11 ... Work device hydraulic actuator (lift cylinder, bucket cylinder), 12 ... Restriction, 13 ... Relief valve, 14 ... Operation lever (operation device), 15 ... Shuttle valve, 16 ... pressure reducing valve, 17 ... shuttle valve, 20 ... steering handle, 50 ... working device, 61 ... wheel

Claims (2)

A variable displacement hydraulic pump,
An open center type first control valve for controlling the flow rate of pressure oil supplied from the hydraulic pump to a hydraulic actuator for driving a working device;
A second control valve for controlling the flow rate of pressure oil supplied from the hydraulic pump to a steering actuator for steering the wheel;
A priority valve for preferentially supplying pressure oil from the hydraulic pump to the second control valve;
A throttle provided downstream of a center bypass line in the first control valve;
In a work vehicle comprising an operating device for changing a spool position of the first control valve,
A work comprising a flow rate control device that controls the discharge flow rate of the hydraulic pump in a range that is smaller than when the operation device is operated and that is capable of driving the steering actuator when the operation device is not operated. vehicle. The work vehicle according to claim 1,
The work vehicle characterized in that the flow rate control device controls a value of a discharge flow rate of the hydraulic pump so as to increase with an increase in an operation amount of the operation device when the operation device is operated.

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